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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 25.0.3
Operating system and version (eg, Ubuntu 20.04): AlmaLinux 8.7
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.37
PHP version (eg, 7.4): 7.4.30
The issue you are facing:
When attempting to use the Talk Android client internally on WLAN I get the message: “failed to connect to https://sub.mydomain.org/publicipaddress” (but, of course, it displays my actual public domain name and public IP address in the the client). I have no issues with Talk within the web browser on my LAN nor using the Android client external of my LAN. In other words, if I turn off my Wi-Fi on my phone and attempt to connect to https://sub.mydomain.org I can connect and login just fine. But when I turn on my Wi-Fi is when I get the connection failure. Weirdness.
I am self-hosting Nextcloud on a web server on my LAN. I have a separate server running Nginx Proxy Manager as a frontend pointing at my web server.
The NPM configuration:
Details Tab:
Domain Names: sub.domain.org
Scheme: http
Forward: web_serverlanip
Forward Port: 80
SSL Tab:
sub.domain.org certificate is selected. Force SSL and HSTS Enabled is also selected
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- While on WLAN, open Talk Android client and enter sub.mydomain.org
- While on WLAN, open Talk Android client and enter sub.mydomain.org
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'x',
'passwordsalt' => 'x',
'secret' => 'x',
'trusted_domains' =>
array (
0 => 'serverlanip',
1 => 'serverhostname',
2 => 'serverfqdn',
3 => 'sub.mydomain.org',
),
'datadirectory' => '/path/to/data',
'dbtype' => 'mysql',
'version' => '25.0.4.1',
'overwriteprotocol' => 'https',
'overwrite.cli.url' => 'http://serverlanip',
'dbname' => 'nextclouddb_name',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextclouddb_username',
'dbpassword' => 'blahblah_Im_not_telling',
'installed' => true,
'updater.secret' => 'x',
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
);
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE
PASTE HERE
Output errors in nextcloud.log in /var/www/ or as admin user in top right menu, filtering for errors. Use a pastebin service if necessary.
PASTE HERE